This invention relates to the field of crystal growth and annealing, specifically a method and apparatus for crystal growth and annealing with minimized residual stress and suitable for production of calcium fluoride (CaF2) crystals.
Crystals are used in a wide variety of applications, including as lenses in digital broadcast cameras and as optical elements in lithography such as in semiconductor processing. Semiconductor lithography at 193 nm wavelengths commonly used fused silica optical elements. Unfortunately, fused silica is damaged by high fluence at 193 nm. The next generation of semiconductor lithography is expected to use 157 nm wavelength illumination. Another material will be required since fused silica is quite opaque to 157 nm wavelength illumination.
CaF2 is one of several candidates for optical elements in 193 nm and 157 nm lithography. Current crystal growth and annealing processes lead to high residual stress in large CaF2 crystals, however, limiting the applicability of CaF2 crystals. High residual stresses in a crystal can cause the crystal to exhibit a spatially varying index of refraction. This can lead to wavefront errors, image degradation, and birefringence, all detrimental to the effectiveness of an optical system using CaF2. Contemporary crystal growth and annealing is illustrated by FIG. 1(a,b). A powder P is placed in a crucible C. During the growth phase, the powder P is heated to a liquid phase (roughly 1500xc2x0 C. for CaF2, for example). The crucible C is slowly lowered from the heated region R1, with the crystal X growing in the region R2 where the liquid can cool below a critical temperature. The difference between the liquid temperature T1 and crystal temperature T2 leads to a temperature gradient across the crystal/liquid combination.
Once the crystal growth phase is complete, the crystal X is annealed. FIG. 1b shows the arrangement in a conventional annealing process. The crystal X is placed back in the heated region R1, but the temperature is less than that required to liquefy the crystal X. The crystal loses heat through its top, bottom, and sides. The temperature of the crystal is slowly reduced until it reaches a certain value, typically room temperature (annealing a CaF2 crystal conventionally takes approximately 30 days to bring the temperature from 1000xc2x0 C. to 50xc2x0 C., at cooling rates of less than 1xc2x0 C. per hour). The temperature of the crystal is slowly reduced, conventionally still with a vertical temperature gradient as represented by differences between T1 and T2. After the crystal is completely cooled, typically the top and bottom are cut off to produce a blank. The blank can then be ground and polished to produce an optical element such as a lens, tube, or plate. Current CaF2 crystal production methods reliably produce CaF2 crystals of limited size, because the CaF2 crystals produced exhibit unacceptably high birefringence at sizes over about 6 inch diameter. The limited size crystals limit the numerical aperture available with resulting optical elements, limiting the optical elements"" utility for high density lithography. Accordingly, there is a need for a method and apparatus for producing crystals that minimizes birefringence even at large crystal sizes, and is suitable for production of CaF2 crystals.
The present invention provides a method and apparatus for producing crystals that minimizes birefringence even at large crystal sizes, and is suitable for production of CaF2 crystals. The method of the present invention comprises annealing a crystal by maintaining a minimal temperature gradient across the crystal while slowly reducing the bulk temperature of the crystal. An apparatus according to the present invention includes a thermal control system added to a crystal growth and annealing apparatus, wherein the thermal control system allows a temperature gradient during crystal growth but minimizes the temperature gradient during crystal annealing.
Advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.